Saturable reactor battery charger



SQL 26,' 1950 c; F. LEATHERS' 2,523,472

sA'ruRABL-J meros BA'rrERY CHARGER Filed Ju1y;1,1. 1947 JIVII caca E. INVENTOR.

Patented Sept. 26, 1950 UNITED STATES .PATENT oi-'ricav SATUBABLE BEACTOR BATTERY CHARGER Chester F. Leathers, Detroit, Mich., assigner to Progressive Welder Company Application July 1l, 1947, Serial No. 760,313

(Cl. 32H9) l 14 Claims.

This invention relates generally to electrical control systems and more especially to an electrical control system for controlling the charging operation of a battery. The battery-charging system is particularly adaptablefor use with battery-powered welding systems such as. for example. is shown in the copending application of Fred H. Johnson and myself, Serial No. 479,998, filed March 22. 1943, now Patent No. 2,452,573, for Welding Method and Apparatus.

The principal objects of the present invention are to provide a. new and improved battery-charging system of the character described; to provide such a battery-charging system which may be continually energized during the use of the battery and which is automatically controlled to maintain the battery in a fully charged state;

Y to provide such a battery charger in which the controlled elements are arranged to provide extremely close regulation of the battery voltage; to provide a charger in which the rate of charging may be regulated between wide limits in accordance with slight' changes in battery voltage; to provide such a battery-charging system in which the charging rate is controlled by a differential voltage which is equal to the difference between a control voltage and the battery voltage; to provide such a charging system in which the control voltage is varied in accordance with the rate of charging current to the battery: to provide means for varying the control voltage in a predetermined relation to changes in the charging rate; to `provide a second means for controlling the charging voltage which is actuateble to reduce the charging voltage during periods when the battery is supplying its load circuit; to provide means for cooling the battery-charging network or control system whereby the charging system may be subjected to prolonged overload periods; and to provide generally improvedelectrical control systems of the .character described.

With the above as well as other objects in view. which appear in the following description and in the appended claims, a preferred but an illustrative embodiment of the invention is shown in the accompanying drawing, throughout the several views of which corresponding reference characters are used to designate corresponding parts, and in which:

Figure l is a schematic view of a battery-charglng system embodying the invention;

Figure 2 is a diagrammatic sectional view showing the cooling meam for the charger; and

Fig. 3.is a partial schematic view oi' a modied form of the invention. v l

It will be appreciated from a complete understanding of the present invention that the improvements thereof are, in a generic sense, applicable to a wide variety of types of electrical control systems. It is now preferred to utilize these improvements in connection with a battery for resistance welding,more particularly resistance spot welding, of various materials and. in an illustrative but not in a limiting sense, the invention is so disclosed herein.

The numeral I designates generally a batterycharging system havingl input terminals 2, 3 and l, adapted to be connected to a three-phase sixty cycle power supply, of for example 440 volts, and having direct current output terminals l and i to which the terminals of a battery B, which isto be charged, are connected. A load circuit I is connected to the battery B through the terminals 5 and t and has a pair of welding electrodes E, the current to which is controlled by suitable switch contacts SWb. A three-phase saturable core-type reactance Il is provided and has three power windings I2, Il and It; one terminal of each thereof being connected to the power-supplying terminals 2, 3 and l, respectively. The other terminal of each of the windings I2, Il and I6 is connected to one terminal of the primary winding of a three-phase Y-connected transformer Il. The Y-connected secondary windingsof the transformer Il are each connected through manners zo, zz and u to the output terminal c" and the common terminals of the secondarywindings are connected to the other terminal l of the charger. y

Current through each of the windings I2, Il and It is controlled respectively by core saturating coils 2B. 2l and ll. which are shown as being parallelly connected and supplied from a suitable source of power through conductors 3l and 34 by means of a full-wave rectifier It. One terminal of each of the saturating coils 2i, Il and Il is connected to one of the direct-current outl put terminals of the rectifier 36 and the other terminals of these coils are connected to the other of the direct-current terminals of the rectiller 3l. A condenser C2 is connected between the direct-current terminals of the rectifier It to smooth out variations in the direct-current output thereof.

The lines I2 and u are connected to and supply the two alternating current input terminals of the rectier Il. Inserted in series in the conductor u and controlling current ilow therethrough are the terminals SWa of the switch 8W. A resistor RI is connected in shunt around the contacts SWa so that when the contacts SWa are open circuited the rectifier 3B will remain energized at a lower voltage. 'I'he switch SW may be controlled by means of a contacter CI controlling current to the energizing winding of the switch SW from a suitable source of supply or may be, as is more usual, one of the better known automatic welding contactar controls such as, for example, as shown in said Johnson and Leathers application Serial No. 47 9,998.

A saturating core-type reactor 38 is arranged in series in the conductor 34 and has a saturatlng control winding 40 having one terminal which may be connected by a conductor 39 having a switch SWI and a variable resistor VR. to the terminal 5 of the charger system and therethrough to one terminal of the battery B. The other terminal of the winding or coil 40 is connected to one of the direct-current output terminals of a full-wave rectifier 42. The other direct-current terminal of the rectifier 42 is connected directly to the output terminal and therethrough to the other terminal of the battery B. The polarities of the terminals and 6 and the direct current output terminals of the rectifier 42 are arranged to provide bucking D.C. voltages whereby current flow through the winding 40 is proportional to the difference between the voltage of the battery B and of the rectifier 42. The alternating-current terminals of .the rectifier system 42 are connected to opposite ends of a secondary winding of a transformer 44. The primary winding of the transformer 44 is connected to one input phase of the three-phase transformer I8. In order that the charger system may be compensated for changes in voltages in the supply connected to terminals 2, 3 and 4, a network generally designated 60 may be provided in the conductor 39 connecting terminal I with the winding 40. This network l0 comprises transformers 62 and 44 adapted to have their secondary windings connected in opposition and in series with a rectifier 66 and a resistance heater 68 in heat exchange relation with a temperature responsive current-controlling resistor 1I arranged in the conductor 39 in shunt relationship with the switch SWI. The primary windings of the transformers 62 and 64 are connected as indicated by the numerals 2 and 3 to the terminals 2 and 3. Transformer I2 is of the usual type in which the voltage in the secondary winding is proportional to that of its primary. The transformer 64 on the other hand is of the constant voltage type in which the output or secondary voltage is substantially constant irrespective of changes in voltage in the primary winding. If desired, the transformer 44 may be replaced by a transformer 64' of the same type as transformer 62 and its primary winding connected to a controlled source of voltage as indicated and which may be controlled in magnitude but which will remain substantially constant at any controlled value.

The rectifiers 20, 22 and 24 are preferably of the usual dry-disc type and these rectifiers 20, 22 and 24, together with the reactor I0 and transformer I8, are preferably located within a fluidtight housing 46 having a water-cooled radiator 48 through which oil. which fills the interior of the housing 46, is propelled by means of a propeller 50 driven by a motor 52 also located within the housing 46. Energization of the motor 52 and a solenoid valve 54 controlling flow of cooling water through the radiator 48 is controlled by a switch SW3 which may be either automatically or manually energized simultaneously with clo- CII 4 sure of the line switches LS supplying the terminals 2, 3 and 4. It will be obvious that the housing 4t could be designed to operate without the propeller III under natural convection flow of the oil therewlthin.

The provision of a totally enclosed fluid-cooled arrangement for the rectiflers is particularly desirable not only in instances such as presently described but in connection with other uses where the atmosphere in which the rectifier may be situated is of a corrosive or explosive nature such as, for example, adjacent an electroplating tank which under present usage requires the rectifiers to be located a substantial distance away and the current to be' conducted thereto through heavy busses which/areexpenslve and absorb energy from the system dueV to their electrical resistance.

It is believed thatthe remainder of the details of this invention may best be understood by reference to the description of operation thereof. When it is desired to place the system I in operation, the usual ilne switches LS and SW3 are closed whereby the terminals 2, 3 and 4 of the charging system I are energized with suitable voltage such as three-phase sixty cycle alternating current and solenoid valve B4 and motor l2 are energized.

A voltage will therefore immediately be applied through the reactor I0 to the conductors l2 and 34 supplying the rectifier 34 and to the transformer 44 supplying the full-wave rectifier 42. Assume the battery is fully charged and the potential across its terminals and, consequently, the terminals I and l, is for example, 2.2 volts or a multiple thereof depending upon the number of series-connected cells of the battery B. For purposes of explanation, the battery will be considered as having one or more units of three seriesconnec'd cells in which the fully charged terminal voltage is 6.60 volts. The battery voltage appearing across the terminals i and 6 of 6.60 volts will have a determined relationship to that appearing across the direct-current terminals of the rectifier 42 so that the current flow through the saturating winding 44 0f the reactor Il is that required to regulate the reactor Il to provide an output voltage of the rectifiers 2l, 22 and 24 of substantially 6.60 volts. In the event it is desired to operate the system with the switch SWI closed and to omit the compensating effect of the network il, the value of this current may be zero. It is to be understood that the rectifier network 42 will prevent current from flowing through the winding 4I in a direction which would occur with the battery voltage B greater t-han the direct-current voltage across the network 42.

The reactor I4 being only partially saturated. or completely unsaturated, the current flowing through the rectifier network 38 and the parallelly connected windings 2i, 23 and 30 supplied thereby will be at a desired value. Therefore, the drop in potential across the reactor III will be sufficient so that the voltage applied to the primary winding of the transformers 44 and Il will be, for example, 350 volts. The voltage applied to the terminals l and 4 will be equal to or just slightly greater than 6.60 volts so that a trickle charge of perhaps 20 amperes, which when compared with the full charging rate of 1000 amperes is very small, may be continually applied to insure a fully charged battery. Under this fully charged battery condition, the voltage across the rectifier network 42 will be just sufficient when compared to the voltage appearing across the genaue 5 terminalslandltopmvidethedesiredcurrent flow in the winding 4I.

Assume that the terminal voltage or the battery B falls to a value of, for example, 6.59 volts. The voltage between the terminals l and i falls with respect to the voltage appearing across the direct-current output terminals of the network 42 and current through the saturating coil 4l will increase.' This increase in current through/the coil 4I will tend to increase saturation of the core oi' the reactor 38 thereby decreasing the voltage drop therethrough and increasing the voltage across the rectifier Il with a consequent increase in current now through the saturating Awindings 2l, 2l and 30 of the reactor Il. This increase in current through the saturating windings of the reactor III will decrease the voltage drop thereacross and increase the voltage applied to the primary winding of the transformer Il, the primary winding of the transformer 44 and the reactor 38. This increased voltage across the primary winding of the transformer 44 will increase the voltage across the direct-current output terminals of the rectier 42, which will still further increase the current flow through the coil 40 and cause a further increase in output voltage of the reactor ill and of the rectiilers 2U, 22 and 24. Also, the increased voltage across the conductors 32 and 34 will increase the current flow through the coils 2s, n andl n thereby adding its effect to still further increase the output voltage of the reactor II.

This cycle of increasing voltage will continue until a balance is reached in which the differential between-the effective direct-current'voltage output of the rectifier 42 and voltage between the terminals 5 and 6 is exactly the value required to maintain these differentials. The system will charge the battery B at the rate vdetermined by this balance until the battery builds up an in creased voltage which raises the potential thereof and decreases the differential in voltage between that at the terminals l and 6 and that at the rectifier 42. The current iiow through the winding 4l then decreases. This decrease in current will increase the voltage drop across the reactor IB thereby decreasing the current flowing in the windings 2i, 2l 'and 30 and increasing the voltage drop across the reactor i0. This decreases the voltage applied to the transformer I8 and therethrough to the terminals 5 and 8 reducing the charging rate. This lowering of the output voltage of the reactor I0 also lowers the output voltage of the rectiiiers 42 and I6. The lowering of the output voltage of the rectifier 36 still further decreases the output voltage of the reactor Il and the lowering voltage of the rectifier 42 provides a lower differential voltage, for causing current iiow through the coil 40 thereby still further lowering the output voltage of the rectifier 38 which results in a further lowering of the output voltage of the reactor ill. This balancing cycle of lowering charging rate and voltages continues until a new balance between the output voltage of the rectier 42 and battery terminal voltage results'at which time the system is again in balance, It will be obvious, since the terminal voltages 5 and 8 will be continually increasing in a gradual manner during periods in which the battery is being charged with no closure of the contacts SWb. that the rate of charging will be continually decreasing with increasing charge in the battery B as the system continually finds new balance points. Conversely, with a decreasing charge of the batteries B, the charging rate will correspond- 6 ingly increase. The system may be said to modulate its charging rate in accordance to the charged condition of the battery B.

The values of the various system components are so proportioned that the reactor Il will fully saturate and the voltage drop thereacross will be a minimum when the battery voltage falls to approximately 6.50 volts. Of course, if it is desired. other voltage ranges may be utilized in keeping with the degree of tolerance in battery voltage which is permissible for the .load supplied thereby by a proper proportioning of the voltage ratio of the transformer 44 and saturating effect of the winding 40. If the voltage ratio of transformer 44 is large, s, relatively small change in battery voltage will initiate a great change in the output voltage of the rectifier 42 so\\that the reactor 3l will be completely saturatedrand substantially full-line voltage will be applied to the transformer il with a slight decrease in charge of the battery B. If a lesser voltage ratio of transformer 44 is provided, a relatively greater change in voltage of battery B will be necessary to saturate the reactor 38. This is true because the voltage cross the terminals 5 and 6 with no change in open circuit voltage of the battery B will increase with increase in charging rate. It is desirable to proportion the change of voltage of the rectifier 42 with respect to the change in voltage between terminals 5 and G so that a balance will be obtained within the range of the control of the reactor 38 for the desired range in battery voltage. In other words, the voltage between the terminals I and 6, plus the voltage drop across the winding 40 and resistor VR, must equal the effective output voltage of the rectifier 42. The charging rate characteristic may be represented by the locus of the intersection of the family of voltage curves representing the relationship between charging rate and voltage between the terminals 5 and 6 for various values of open-circuit battery voltages with a curve representative of the voltage output of rectifier 42 less the sum of the voltage drops across the winding 40 and resistor VR. This relationship may be made to provide various charging rates depending upon the setting of the resistor VR which in many respects will act much in the same manner as a change in the turn ratio of the transformer 44.

'I'he foregoing operation has been discussed as if the switch SWI were closed and the voltage supply to terminals 2, 3' and 4 were constant. Let us now consider the operation in which the voltage varies and the switch SWI is open thereby connecting the control resistor l0 into the circuit of the winding 40. The value of the resistor VR is adjusted to provide the desired charging rate. If the voltageof terminals 2 and 3 is say 440 volts, the voltage of the secondary winding of transformer 42 will be `440 volts assuming a 1-1 ratio, although it is to be understood that other ratios may be used. 'I'he secondary voltage of the transformer 64 will be a controlled voltage of say 400 volts whereby a diierential voltage of 40 volts is Il at the terminals 2, 3 and 4 to the normal value.

When the voltage at terminals 2 and l falls say to 420 volts, the diierential in voltage between the secondary windings of the transformers 82 and 64 decreases to 20 volts and the current iiow through, and the temperature of, the resistor il will fall. This decrease in temperature of the resistor 68 will lower the temperature of the resistor thereby decreasing the potential diil'erence between the rectifier 42 and terminals 5 and 6 necessary to maintain current flow through the winding 40. Likewise, an increase in voltage at terminals 2 and 3 will increase the voltage difterential controlling flow through the resistor El, causing its temperature to increase and increase the temperature and resistance of the resistor 10. 'I'his increase in value of resistor 10 raises the voltage differential requirements between rectifier 42 and terminals 5 and 6 to providethe same current ow through the coil 40. If now the effect of the resistor 10 is balanced against the effect of a. change in voltage at the terminals 2, 3 and 4 it will compensate for changes in line voltage and the battery will be charged as if no change in line voltage had occurred. It should be here noted that when the network 60 is used, the conductor 39 and winding 40 should always be subjected to at least a minimum current so that the effect of changes in the value of the resistor 10 will eilect the current ilow through the coil 40.

Now that the theory of operation and arrangement of the apparatus has been set forth, those skilled in the art may now, by the use of the normal design material made use of by designers of electrical apparatus, select and/or design the specific apparatus which will provide a charging rate having the desired characteristics for the particular batteries utilized and for the particular voltage characteristics which may be required for the load circuit with which it may be utilized. It will be appreciated that since the effect of the network 60 is to compensate the control circuit through the winding 40 for changes in line voltage, the transformer 44 could be connected to a constant source of potential ii' such were available. In this event, the transformer 44 would not act to progressively increase the voltage differential between that of the rectifier 42 and the terminals 5 and 6 but a similar effect will be produced by the increasing voltage across the conductors 32 and 34 which will provide a progressively increasing current now in the windings 26, 2B and 30 to progressively increase the voltage supplied to the transformer il until a balance similar to that before described is attained.

Under conditions of operation in which the voltage between terminals 5 and 6 will be materially reduced when a load such as applicant's welding load is connected thereto, it may be desirable to prevent the normal functioning of the charger during the duration of these loads. Un-

, der other circumstances, the duration of the load and the reduction in voltage of the terminals of the battery B is sufiiciently short as compared to the time required to adjust the charger mechanism i, and no great increase in charging current accompanies the connection of the load to the battery B even though an adjustment of the charger control is not made. Since the time required to effect a control in the charger system i may vary considerably depending upon the various time constants of the reactors and transformers involved, it is now considered most desirable to readjust the charging mechanism l to prevent an undesired increase in charging current during intervals when the electrodes E are energized. The switch contacts SWa are arranged to open during closure of the contacts SWb whereby the resistor RI acts to reduce the voltage supplied to the rectiiier 3G. 1f the current flowing through the conductor 34 at the time the load is applied is small, as occurs with the batteries B substantially fully charged, the voltage to the reactor 36 and across the windings 2l, 2l and Il will not drop appreciably and the opening of the contacts SWa will have no appreciable immediate effect. As. however, the voltage between terminals i and i drops, the diilerential in voltage between terminals 5 and i and the rectiner 42 increases and the resistor Rl will limit the increase in voltage at the rectifier Il. When, however, the charge of the batteries B is low, substantial current is delivered by the load circuit to the battery B and a substantial current will be ilowing in the line I4. Under these conditions, the opening of the contacts SWa very quickly reduces the voltage applied to the rectiiier 3l and to the reactor 3l because of the voltage drop through the resistor RI, which reducesthe D.-C. voltage of the rectifier 's6 and lowers the now of current through the windings 28, 2l and 3l. The reactance of the reactor Il will increase thereby reducing the charging voltage so that the charging current iiowing through the rectifiers 20, 22 and 24 to the battery B is not unduly increased due to drop of the terminal voltage of the battery B.

It will be apparent that for every open-circuit voltage of the battery B a predetermined charging current will be supplied thereto; that with the voltage effect produced by connecting the transformer 44 and lines I2 and 34 to the changing output voltage of the reactor i0 a small change in battery voltage may be made to cause a large change in charging current; that such a connection may be made to just balance the increase in terminal voltage of the battery due to the charging thereof so that with the reactor Il. which is designed to provide a voltage change in direct proportion to .change in current ilow through its saturating winding 40, a charge rate may be applied to the battery in direct proportion to its decrease in open-circuit terminal voltage; and that compensation may be provided for changes in voltage of the electrical energy supply.

What is claimed and is desired to be secured by United States Letters Patent is as follows:

1. In a battery-charging mechanism, a nrst pair of terminals adapted to be connected to a battery to be charged, a second pair of terminals adapted to be connected to a source of alternating current power, means interconnecting said pairs of terminals including a reactor having output terminals and a rectifier, said rectifier being supplied with alternating current from said second pair of terminals through said reactor output terminals, means controlling the eil'ective reactance of said reactor whereby the voltage at said rectiner may be regulated, means including take-off conductors connected to and deriving a control voltage from said reactor output terminals, said controlling means including means responsive to the diierence in voltage between that appearing across said nrst pair of terminals and said control voltage, and means responsive to changes in voltage at said second pair of terminals for varying the response of said dinerential voltage responsive means to changes in said diiierence in voltage, said control voltage being asaasva Q proportional to the voltage ot said reactor output terminals.

2. In a battery-charging mechanism, a firstV pair of terminals adapted to be connected to a battery, a second pair of terminals adapted to be supplied with alternating current power. circuit means including rectifylng means interconnecting said pairs of terminals whereby direct current power may be supplied to said rst pair of terminals, means controlling the voltage supplied to said first pair of terminals, and means regulating said voltage-controlling means, said regulating means including means responsive to the differences of two opposing voltages, one of said opposing voltages being proportional to the voltage appearing between said nrst pair of terminals. the other oi' said opposing voltages being proportional to the voltage delivered to said rectifying means, and means responsive to changes in magnitude of the voltage at said second pair of terminals for compensating said regulating means for such changes in terminal voltage.

3. In a battery-charging mechanism, a i'irst pair of terminals adapted to be connected to a battery, a second pair of terminals adapted tc be supplied with alternating current power, circuit means including rectifying means interconnecting said pairs of terminals whereby direct current power may be supplied to said first pair of terminals,means controlling the voltage supplied to said ilrst pair of terminals, means regulating said voltage-controlling means. said/regulating means including means responsive to the` differences of two opposing voltages, one of said opposing voltages being proportional to the voltage appearing between said iirst pair of termi-y nals, the other of said opposing voltages being derived from a transformer energized by a voltage proportional to the voltage delivered to said rectifying means, and means for selectively controlling the response of said regulating means to changes in said differential voltage.

4. In a battery-chargingmechanism, a first pair of terminals adapted to be connected to a battery, a second pair of terminals adapted to be connected to a source of alternating current power, means electrically connecting said pairs of terminals and including a voltage-controlling means and a rectifier. said voltage-controlling means being intermediate said second pair of terminals and said rectifier, regulating means including means responsive to the voltage of the battery connected to said ilrst pair. of terminals and means responsive to the output voltage of said voltage controlling means, said voltage-responsive means being arranged relative to each other to produce opposing eii'ects on said voltagecontrolling means. said last-named voltage responsive means being arranged to provide an efiect which increases proportionally with an increasing output voltage ofvsaid voltage controlling means, said regulating means being responsive to the diil'erential eifect between the ell'ect produced b v said ilrst-named and said secondnamed voltage-responsive means for adjusting the voltage supplied to said rectiner.

5. In a battery-charging mechanism, a ilrst pair of terminals adapted to be connected to a battery, a second pair of terminals adapted to be connected to a source of alternating current power, means electrically connecting said pairs of terminals and including a voltage-controlling means and a rectifier, said voltage means being intermediate said second pair of terminals and said rectiiler, regulating means for adjusting the voltage supplied to said rectier including means responsive to the voltage of the battery cmnectedtosaidnrstpairofterminabandmea responsive to the output voltage of said voltage means, said voltage-responsive means being anranged relative to each other to produce opposing eilectson said voltage regulating means. all! means for.. selectively controlling the eilect lloduced on said regulating means by said oppoaim eilects.

6. In a battery-charging mechanism, a mat pair of terminals adapted to be connected to a battery, a second pair of terminals adapted to be connected to a source of alternating current power, a saturable core-type reactor having a main winding and a saturating winding. a power transformer having a primary winding and a secondary winding, said primary winding being connected across said second pair of terminal and in a series with said reactor main winding, a rectifier means having one terminal connected to one end of said secondary winding and having its other terminal connected to one of said ill-st pair of terminals, the other of said iirs't pair of terminals being connected to the other end of said secondary winding, a control circuit adapted to be energized with alternating current and including a rectiiler means having its direct current terminals connected to supply direct current to said saturating winding, a saturable reacts! controlling flow of current through said control circuit and having a saturating coil, and a tramformer having its primary winding connected in parallel with said power transformer primary Y winding, said last-named transformer having a vsaid series connected saturating coil and direct current output terminals being connected to said rst pair of terminals whereby the current ilo' through said saturating coil is proportional to the difference in voltage appearing across said nrst pair of terminals and that appearing acres said direct current output terminals.

7. In a battery-charging mechanism, a first pair of terminals adapted to be connected to a battery, a second pair of terminals adapted to be connected to a source of alternating current power, a saturable reactor deriving current troni said second pair of terminals and having a sata'- rating coil and output terminals, a rectier ccnnected to said reactor terminals and delivering direct current power to said first pair of terminals, means for supplying said coil with direct current, a transformer having a primary and a secondary winding, means connecting said xximary winding with said reactor output termirnls, full-wave rectier means connected to be supplied from said secondary winding, circuit meer' interconnecting the direct-current output of sail rectifier means in opposition to the voltage ap'- pearing across said first pair of terminals, mea!! responsive to the diierence in voltage acro said first pair of terminals and across said direct-current including a source of control potential and a source of potential the magnitude of which is proportional to the magnitude of said mst-named source, and means responsive to the differential in voltage between said control potential and said last-named potential for regulating said current iiow.

9. The combination of claim 8 in which said ilrst-named source comprises two opposed voltages and said differential responsive means comprises a resistance heater element in heat exchange relation with a resistor arranged in series circuit in said network with said first-named responsive means.

10. 'I'he combination of claim 8 in which said constant potential is provided by a constant potential output transformer supplied with energy from said first-named source.

11. In a battery-operated heating system, a heating circuit receiving electrical energy from the battery. a charging network for the battery comprising conductors adapted to be connected to a source of alternating current energy and operatively connected to the battery, rectifying mearm associated with said conductors whereby unidirectional current is supplied to charge the battery, reactance means controlling the ilow oi current to the battery. means responsive to the charged condition of the battery for controlling sald reactance means, and means responsive to the energization of said heating circuit for controlling said reactance means.

12. In an electrical network for supplying a heating circuit, a first pair oi` terminals operable to supply electrical energy to the heating circuit, an electrical energy storage device having output conductors connected to said terminals, a second pair of terminals adapted to be secured to a source o! alternating current electrical energy, a saturable reactor, a transformer, a rectier, circuit means connecting said reactor and said transformer and said rectifier in series arrangement container.

CHESTER P'. LEATHERS.

REFERENCES CITED The following references are of record in the ille of this patent:

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